Recently, the carbon fiber has been utilized for advanced composites of plastics, metals or ceramics based on its superior mechanical properties, such as high strength, high modulus and low specific gravity. In particular, carbon fiber reinforced plastics have been practically utilized for various applications, for example, in aerospace planes, automobiles, industrial machines, the leisure industries and others.
In such applications, much higher performance and strength of the carbon fiber has been desired. The term "fiber", as used herein, signifies a continuous long fiber. The carbon fiber previously had a tensile strength of about 300 Kg/mm.sup.2 but recently has been improved up to a level of 400 Kg/mm.sup.2. Nowadays, a higher strength of 500 Kg/mm.sup.2 is required.
However, the carbon fiber having a tensile strength of 500 Kg/mm.sup.2 can not be readily prepared by conventional improved methods. And, the commercially available carbon fiber of 400 Kg/mm.sup.2 can not give its full performance when used as a composite material.
There is a known process in which acrylonitrile is polymerized in an aqueous concentrated zinc chloride solution to form a polymer solution which is then spinned into an aqueous dilute zinc chloride solution to prepare an acrylic fiber. Practically, in the known process, 5 to 10% of sodium chloride is added to the polymer solution in order to reduce its viscosity. However, the presence of a non-solvent, such as sodium chloride, in the solution decreases stringiness of the solution, resulting in difficulty of obtaining each filament of small diameter. Such a system for producing a carbon fiber from the acrylic fiber is disclosed in Japanese Patent Publication No. 39938/77.
Further, there has been used a process for preparing the acrylic fiber and the carbon fiber from polyacrylonitrile solution in an organic solvent, such as dimethyl-formamide or dimethylsulfoxide. In this process, however, the single fiber filament of the carbon fiber thus prepared has a somewhat flat cross-section and is difficult to free from the organic solvent. A carbon fiber of high strength can not be obtained (its tensile strength is at most 350 Kg/mm.sup.2).
Accordingly, an object of the invention is to provide a carbon fiber having a tensile strength of more than 400 Kg/mm.sup.2 and the ability of giving a composite material of high strength.
The conventional methods have utilized various techniques for improving the performance of the composite material, for e.g., (a) preventing incorporation of foreign substances into a precusor during the spinning step or (b) by coating a filament surface with an oil agent for preventing agglutination during the stabilizing and carbonizing steps. This prepares the carbon fiberfree of defects. It is then subjected to surface treatment for improving wettability to plastics. It has now been determined that a carbon fiber of high strength may be obtained by using a suitable precursor, and that the carbon fiber having ruggedness on its surface may improve compatibility to a matrix for giving it full performance in use as a composite material.
As a result of the continued search for obtaining a suitable polyacrylonitrile (PAN) precursor for the carbon fiber from a standpoint other than clothing fibers, it has now been determined that the defects in the clothing fiber, such as devitrification and fibrillization, may have positive advantages for the carbon fiber precursor.
Further, as a result of studying the process for preparing the carbon fiber of high strength in the zinc chloride system, it has now been determined that, without the addition of a non-solvent salt the zinc chloride system together with the lower polymer concentration and the higher draft ratio (in the presence of the non-solvent the lower polymer concentration cannot provide the high draft ratio) may provide a single filament having a diameter of less than 10 .mu.m, and which results in the carbon filament of high strength. In this case, an aperture length/diameter (L/D) ratio of a spinning nozzle of more than 2 may facilitate increase of the draft ratio.